Replace ice when it’s mostly melted.

That’s the simple rule. Temperature danger zone for food is 40°F–140°F. Once the ice turns to water, the water temperature climbs fast – especially if you’re opening the cooler a lot.

Block ice lasts way longer than cubes. Cubes have more surface area, so they melt quicker. If you’re using cubes for a day trip, expect to replace them every 4–6 hours. Block ice can last a full day or more in a decent cooler.

Don’t overthink it. If the water feels warm to the touch, it’s too warm. If food starts floating in lukewarm water, change the ice. Your cooler’s insulation matters too – a cheap cooler loses ice fast. Invest in a good one if you do this often.

Common sense beats a timer.